The overall objective of this research project is to identify and characterize the actions of brain neurotransmitters that regulate the secretion of oxytocin (OT) from the neurohypophysis, and especially, those neurochemical systems that are involved in the release of the peptide in response to suckling during lactation. The proposed research will test aspects of the overall hypothesis that norepinephrine (NE) released within the hypothalamic magnocellular nuclei interacts post-synaptically with neuropeptide Y (NPY), which may be co- released as a peptidergic co- transmitter, with an excitatory amino acid, most likely glutamate (Glu) acting at an AMPA receptor subtype, and with inhibitory GABAergic systems, to produce the characteristic episodic activation of OT secretion during lactation. We have also obtained evidence that during the early stages of lactation, afferent stimuli provided by the suckling offspring participate in the up-regulation of OT mRNA expression, in part by increasing the local release of OT in the magnocellular nuclei. The first Specific Aim uses a combination of electrical stimulation, neuropharmacological, and in vivo microdialysis approaches to test whether activation or disruption of transmission through specific noradrenergic cell groups of the brain stem alters the central release of NE and systemic release of OT in response to suckling. The second Specific Aim is to further examine the interactions among NE, NPY, Glu and GABA in control of OT secretion. Studies will test whether AMPA receptors control NE or NPY release in the magnocellular nuclei and whether GABA systems modulate the OT secretory response to excitatory inputs. Whole-cell electrophysiological recordings will be made from immunohistochemically identified OT neurons in slices of the supraoptic nucleus in vitro to examine the effects and interactions between NE and Glu on activity of OT neurons. The third Specific Aim will further investigate the neuroendocrine regulation of hypothalamic OT mRNA expression in lactation. In situ hybridization will be used to determine whether the physiological stimuli associated with suckling alter OT mRNA in discrete subsets of OT neurons. To further examine the involvement of centrally released OT in OT mRNA expression, studies will test whether centrally applied OT increases hypothalamic OT mRNA levels in non-lactating rats, and microinjection approaches will be used to determine the neural site of action of OT in increasing OT mRNA levels. Studies with NE and GLu receptor antagonists will test whether OT mRNA levels in the magnocellular nuclei are regulated by a noradrenergic-Glu interaction as is OT release.